Quantized feedback may be used in control loops to perform analog to digital conversion. Analog to digital converters (ADCs) with such features are often known as sigma-delta (ΣΔ) converters, or ΣΔ modulators, the modulator term referring to an output digital data stream having a certain symbol pattern, or modulation, imposed upon it by the control loop. The terms ΣΔ modulator and noise shaping control loop are often used interchangeably in the art, although the latter is more descriptive. Circuit designers often like to use such ΣΔ modulators as in many cases they may be simpler to design and cheaper to make than other types of ADCs.
In such a noise shaping control loop, a continuous analog signal is applied at the input, and a digital pattern representative of this signal emerges from the output. The digital signal is created by one or more quantization elements in the control loop, for example, by non-linear elements in the loop such as flip-flops or comparators that have a discrete set of non-continuous output values for any given continuous input quantity.
The ΣΔ modulation works by constraining a feedback parameter to one of a set of at least two specific values, and a control loop of arbitrary order ensures that the average feedback value equals the input. Instantaneous deviations from the ideal continuous feedback necessarily introduced by quantization elements represent noise, and a sophisticated, possibly high order, control loop can suppress or “shape” this noise. To “shape” the noise means to filter it, generally to make it not appear in certain frequency bands. The loop therefore operates to suppress this noise in certain frequency bands of interest, often at the expense of increased noise in bands that are not relevant to the application. Hence ΣΔ modulators are sometimes also referred to a “noise shaping loops.”
In some ΣΔ modulators the input parameter is a current (or a difference in currents), in part of the circuit, and two or more currents (or difference currents) are generated to provide feedback. FIG. 1 shows an example of a circuit 100 of the prior art, in which an input difference current is made to flow in the drains of transistors M1 and M2. In this case, the “common mode current” must be balanced, i.e., the sum of the currents through the drains of M1 and M2 must equal the sum of the currents in current sources I3, I4 and I5.
The common mode control portion of the circuit 100, i.e., the components below transistors M1 and M2, adds complexity and cost. Further, whenever a circuit operates by nulling out the difference of two quantities, as in circuit 100, it is only the nominal value of those quantities that are caused to be equal; if the quantities have associated noise, that noise is not cancelled, but rather remains in the difference of the two quantities as the root-sum-square of the individual noise components.
For these reasons, a simple and inexpensive way of improving the performance of ΣΔ modulators by eliminating any common mode control may be useful.